The present invention relates to a mass spectrometer and a method of mass spectrometry.
A known method of obtaining a mass spectrum is to record the output signal from an ion detector of a mass analyser as a function of time using a fast Analogue to Digital Converter (ADC). It is known to use an Analogue to Digital Converter with a scanning magnetic sector mass analyser, a scanning quadrupole mass analyser or an ion trap mass analyser.
If a mass analyser is scanned very quickly for a relatively long period of time (e.g. over the duration of a chromatography separation experimental run) then it is apparent that very large amounts of mass spectral data will be acquired if an Analogue to Digital Converter is used. Storing and processing a large amount of mass spectral data requires a large memory which is disadvantageous. Furthermore, the large amount of data has the effect of slowing subsequent processing of the data. This can be particularly problematic for real time applications such as Data Dependent Acquisitions (DDA).
Due to the problems of using an Analogue to Digital Converter with a Time of Flight mass analyser it is common, instead, to use a Time to Digital Converter (TDC) detector system with a Time of Flight mass analyser. A Time to Digital Converter differs from an Analogue to Digital Converter in that a Time to Digital Converter records just the time that an ion is recorded as arriving at the ion detector. As a result, Time to Digital Converters produce substantially less mass spectral data which makes subsequent processing of the data substantially easier. However, one disadvantage of Time to Digital Converters is that they do not output an intensity value associated with an ion arrival event. Time to Digital Converters are, therefore, unable to discriminate between one or multiple ions arriving at the ion detector at substantially the same time.
Conventional Time of Flight mass analysers sum the ion arrival times as determined by a Time to Digital Converter system from multiple acquisitions. No data is recorded at times when no ions arrive at the ion detector. A composite histogram of the times of recorded ion arrival events is then formed. As more and more ions are added to the histogram from subsequent acquisitions, the histogram progressively builds up to form a mass spectrum of ion counts versus flight time (or mass to charge ratio).
Conventional Time of Flight mass analysers may collect, sum or histogram many hundreds or even thousands of separate time of flight spectra obtained from separate acquisitions in order to produce a final composite mass spectrum. The mass spectrum or histogram of ion arrival events may then be stored to computer memory.
One disadvantage of conventional Time of Flight mass analysers is that many of the individual spectra which are histogrammed to produce a final mass spectrum may relate to acquisitions wherein only a few or no ion arrival events were recorded. This is particularly the case for orthogonal acceleration Time of Flight mass analysers operated at very high acquisition rates.
Known Time of Flight mass analysers comprise an ion detector comprising a secondary electron multiplier such as a microchannel plate (MCP) or discrete dynode electron multiplier. The secondary electron multiplier or discrete dynode electron multiplier generates a pulse of electrons in response to an ion arriving at the ion detector. The pulse of electrons or current pulse is then converted into a voltage pulse which may then be amplified using an appropriate amplifier.
State of the art microchannel plate ion detectors can produce a signal in response to the arrival of a single ion wherein the signal has a Full Width at Half Maximum of between 1 and 3 ns. A Time to Digital Converter (TDC) is used to detect the ion signal. If the signal produced by the electron multiplier exceeds a predefined voltage threshold then the signal may be recorded as relating to an ion arrival event. The ion arrival event is recorded just as a time value with no associated intensity information. The arrival time is recorded as corresponding to the time when the leading edge of the ion signal passes through the voltage threshold. The recorded arrival time will only be accurate to the nearest clock step of the Time to Digital Converter. A state of the art 10 GHz Time to Digital Converter is capable of recording ion arrival times to within ±50 ps.
One advantage of using a Time to Digital Converter to record ion arrival events is that any electronic noise can be effectively removed by applying a signal or voltage threshold. As a result, the noise does not appear in the final histogrammed mass spectrum and a very good signal to noise ratio can be achieved if the ion flux is relatively low.
Another advantage of using a Time to Digital Converter is that the analogue width of the signal generated by a single ion does not add to the width of the ion arrival envelope for a particular mass to charge ratio value in the final histogrammed mass spectrum. Since only ion arrival times are recorded the width of mass peaks in the final histogrammed mass spectrum is determined only by the spread in ion arrival times for each mass peak and by the variation in the voltage pulse height produced by an ion arrival event relative to the signal threshold.
However, an important disadvantage of conventional Time of Flight mass analysers comprising an ion detector including a Time to Digital Converter detector is that the Time to Digital Converter detector is unable to distinguish between a signal arising due to the arrival of a single ion at the ion detector and that of a signal arising due to the simultaneous arrival of multiple ions at the ion detector. This inability to distinguish between single and multiple ion arrival events leads to a distortion of the intensity of the final histogram or mass spectrum. Furthermore, an ion arrival event will only be recorded if the output signal from the ion detector exceeds a predefined voltage threshold.
Known ion detectors which incorporate a Time to Digital Converter system also suffer from the problem that they exhibit a recovery time after an ion arrival event has been recorded during which time the signal must fall below the predetermined voltage signal threshold. During this dead time no further ion arrival events can be recorded.
At relatively high ion fluxes the probability of several ions arriving at the ion detector at substantially the same time during an acquisition can become relatively significant. As a result, dead time effects will lead to a distortion in the intensity and mass to charge ratio position in the final histogrammed mass spectrum. Known mass analysers which use a Time to Digital Converter detector system therefore suffer from the problem of having a relatively limited dynamic range for both quantitative and qualitative applications.
In contrast to the limitations of a Time to Digital Converter system, multiple ion arrival events can be accurately recorded using an Analogue to Digital Converter system. An Analogue to Digital Converter system can record the signal intensity at each clock cycle.
Known Analogue to Digital recorders can digitise a signal at a rate, for example, of 2 GHz whilst recording the intensity of the signal as a digital value of up to eight bits. This corresponds to an intensity value of 0-255 at each time digitisation point. Analogue to Digital Converters are also known which can record a digital intensity value at up to 10 bits, but such Analogue to Digital Converters tend to have a limited spectral repetition rate.
An Analogue to Digital Converter produces a continuum intensity profile as a function of time corresponding to the signal output from the electron multiplier. Time of flight spectra from multiple acquisitions can then be summed together to produce a final mass spectrum.
An advantageous feature of an Analogue to Digital Converter system is that an Analogue to Digital Converter system can output an intensity value and can therefore record multiple simultaneous ion arrival events by outputting an increased intensity value. In contrast, a Time to Digital Converter system is unable to discriminate between one or multiple ions arriving at the ion detector at substantially the same time.
Analogue to Digital Converters do not suffer from dead time effects which may be associated with a Time to Digital Converter which uses a detection threshold. However, Analogue to Digital Converters suffer from the problem that the analogue width of the signal from individual ion arrivals adds to the width of the ion arrival envelope. Accordingly, the mass resolution of the final summed or histogrammed mass spectrum may be reduced compared to a comparable mass spectrum produced using a Time to Digital Converter based system.
Analogue to Digital Converters also suffer from the problem that any electronic noise will also be digitised and will appear in each time of flight spectrum corresponding to each acquisition. This noise will then be summed and will be present in the final or histogrammed mass spectrum. As a result; relatively weak ion signals can be masked and this can lead to relatively poor detection limits compared to those obtainable using a Time to Digital Converter based system.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.